High temperature wire and cable insulation, in particular insulation for aerospace wire and cable, requires a combination of excellent electrical properties, resistance to cut-through and abrasion, fire retardance, low smoke generation, safety in use, ease of stripping and termination, and resistance to chemicals and solvents. High temperature wire and cable insulation based on fluoropolymers, such as PTFE and copolymers of TFE with hexafluoropropylene, such as Teflon.RTM. FEP, or with perfluorovinyl ethers, such as Teflon.RTM. PFA, are well known in the art. While such insulation has excellent chemical resistance to fluids with which it may come into contact, such as "Skydrol.RTM.," and possesses outstanding dielectric and weathering properties as well, its mechanical properties are such that it frequently offers little resistance to scrape abrasion and exhibits less than desirable resistance to cut-through, particularly at the elevated temperatures associated with its use in avionic systems which specify operation at 180.degree. C. and can encounter excursions to 200.degree. C.
In order to address this mechanical deficiency at elevated temperatures, insulating composites have been devised which incorporate polyimide resins as well as fluoropolymers. Such insulation is frequently based on polyimide films such as Kapton.RTM., which are initially coated or laminated with TFE copolymers such as FEP or PFA, e.g., Kapton.RTM. F. These films are then slit into tapes which are self-adhering while they are wrapped on to conductors at elevated temperature. These wrapped tapes may then be taped over or extruded over with fluoropolymers to provide a wire insulation whose exterior exhibits the good chemical resistance, weatherability, and dielectric behavior of the fluoropolymer, and whose polyimide interior provides the mechanical toughness to offer good scrape abrasion and cut-through resistance at elevated temperatures.
While constructions containing polyimide are thought to have a very desirable balance of properties since the polyimide has very good high temperature properties, excellent fire retardance and very good electrals, it has been determined that under some conditions polyimide-containing constructions can arc track, resulting in the formation of a carbonaceous char. The resulting conductive path can rapidly propagate when seemingly small voltages are applied. Arc tracking is a catastrophic failure in the presence of an electrical arc when a short circuit occurs between the conductor and a conducting medium external to the insulation such as a moderately conductive fluid. Such a failure may be occasioned by relatively slight mechanical damage to the insulation which becomes rapidly enlarged at the elevated temperature of the electrical arc.
Fluoropolymers and fluoropolymer-containing materials have excellent electrical properties, including a very low tendency to arc track. However, fluoropolymers are generally soft and prone to cut-through and creep particularly at the elevated temperatures where the use of insulated constructions in avionics is desired.
Many such hybrid constructions of polyimides and fluoropolymers contain layers of TFE copolymers exclusively. However, the presence of a fluoropolymeric layer in the taping elements which is exclusively a TFE copolymer, such as FEP or PFA, results in a tape wrapping element which can be susceptible to delamination at elevated temperatures when only modest adhesion between the polyimide and fluoropolymer is achieved as in the current state of the art. A further short-coming of known polyimide insulation based on tapes employing bonding layers which contain exclusively TFE copolymers is that such insulation does not consistently exhibit good resistance to arc tracking.
The bond between the polyimide and FEP or PFA directly applied to the polyimide in present films such as Kapton.RTM. F is not as fully developed as it could be, being typically about 4 lb/inch at room temperature. Since these copolymers begin to soften and flow even below their melting point, the failure mechanism at the interface for these known films is adhesive and peeling can be initiated at relatively low temperatures compared to their melting temperatures.
Other polyimide/fluoropolymer based tapes such as DF2929 or DF2919 have been commercially available from Chemical Fabrics Corporation. While these tapes contain PTFE homopolymer and a topcoat of PTFE copolymer, such as FEP or PFA, over the polyimide, they do not exhibit the very high adhesion developed between the polyimide and fluoropolymer as do the invention films.
U.S. Pat. No. 3,616,177 describes an asymmetric laminar structure comprising polyimide coated on one or both sides with an FEP copolymer and further coated on one side with a PTFE polymer. However, such a structure is prone to delamination at high temperatures due to the use of a TFE copolymer (FEP) directly on the polyimide layer with only modest adhesion.
U.S. Pat. No. 4,628,003 discloses a heat sealable, high temperature insulative wrapping. The wrapping is a film comprised of a polyimide coated with two fluoropolymer layers. Various fluoropolymers, such as hexafluoropropylene/tetrafluoroethylene copolymers and perfluoroalkyvinylether and perfluoropropylvinylether copolymers with tetrafluoroethylene, are useful in the fluoropolymer layers. Each fluoropolymer layer also contains a pigment to provide coloration and act as an infrared absorber, and also results in increased retention of heat seal bond strengths after heat aging. The polyimide film may also be treated with a silane to improve adhesion of the fluoropolymer layers. These laminate compositions, however, do not entirely avoid the serious problems occasioned by modest interfacial adhesion, particularly at the polyimide/fluoropolymer interface.
Accordingly, it is an object of the present invention to provide improved polyimide and fluoropolymer films for making insulation tape with excellent adhesive and cohesive properties at high temperatures as a means of improving cut through resistance.
It is also an object of the present invention to reduce or eliminate the tendency of known laminations made using polyimide and fluoropolymer tapes to arc track by maximizing the PTFE content of the total fluoropolymer present.
It is a further object of the present invention to provide an improved laminated composite as insulation for wire and cable with excellent properties, as well as a method of making such a laminated composite and related wire and cable constructions.